The effectiveness and safety of parenterally administered ceftazidime were assessed in 20 patients with infections caused by Pseudomonas aeruginosa or Serratia marcescens. There were six infections involving the urinary tract, six wound infections, one respiratory tract infection, three septicaemias, one empyema, one mastoiditis and two infections of the epididymis. Sixteen of the isolates were resistant to at least one aminoglycoside antibiotic. Fifteen patients were clinically and bacteriologically cured, two patients improved, but had relapses and three patients showed no response. The lack of clinical response was due to the development of resistance by P. aeruginosa in two patients with osteomyelitis, one of whom also had diabetes mellitus. The third patient who had chronic lymphocytic leukaemia developed post-operative osteomyelitis and septicaemia with serratia. She did not respond to treatment. Side effects consisted of a positive direct Coomb's test without evidence of haemolysis in two patients and a mild maculopapular rash in one patient.